Compact microwave termination and uses thereof

ABSTRACT

There is disclosed a compact termination for enclosed wave propagation media, particularly waveguide, in which the propagation medium is loaded with an absorptive material throughout a section terminating in a reflective member. The absorptive material or loading material is shaped into first and second portions having impedances lower than the characteristic impedance of the propagation medium and respective lengths in the direction of wave propagation, these impedances and lengths together, providing negligible reflection into the unloaded medium. The loaded section is rendered shorter than one-quarter wavelength by making the portion that is next to the reflective end member have a value of impedance intermediate the impedance of the other portion and the characteristic impedance of the unloaded medium. The direction of stepping of the impedances in this termination is the reverse of that of prior art stepped terminations. The idea is extended to compact waveguide terminations that permit better heat-sinking and to a termination for a coaxial cable.

United States Patent 1 Mohr [ Nov. 5, 1974 1 i COMPACT MICROWAVETERMINATION AND USES THEREOF [75] Inventor: Theodore Warren Mohr,Whitehall,

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, Berkeley l3 CIRCULATOR ISQZATOR HEAT SINKCORE 23 @IEBMI NATION Y Primary Examiner-Paul L. Gensler [57] ABSTRACTThere is disclosed a compact termination for enclosed wave propagationmedia, particularly waveguide, in which the propagation medium is loadedwith an absorptive material throughout a section terminating in areflective member. The absorptive material or loading material is shapedinto first and second portions having impedances lower than thecharacteristic impedance of the propagation medium and respectivelengths in the direction of wave propagation, these impedances andlengths together, providing negligible reflection into the unloadedmedium. The loaded section is rendered shorter than one-quarterwavelength by making the portion that is next to the reflective endmember have a value of impedance intermediate the impedance of the otherportion and the characteristic impedance of the unloaded medium. Thedirection of stepping of the impedances in this termination is thereverse of that of prior art stepped terminations. The idea is extendedto compact waveguide terminations that permit better heat-sinking and toa termination for a coaxial cable.

12 Claims, 11 Drawing Figures 71 TERMINATION WAVE PROPAGATlONDlSSlPATIVE 1 MAT7ER1AL HEAT SINK a5 DISSIPATIVE COAXIAL CABLE STUBPATENTEUxnv 51914 11846320 I3 CIRCULATOR HE g S INKJ I-\-;

ffw l2 TERMINATION I l ISOLATOR PATENTEUHUV 51914 3.846320 sum w T?PAIENTEDHUII 5' I914 3.846720 SIIEEI 701' 7 FIG. /0

7I TERMINATION WAVE PROPAGATION 73 F -77 DISSIPAT MATER F/G. II

DISSIPATIVE MATERIAL 82 8| TERMINATION WAVE PROPAGATION COMPACTMICROWAVE TERMINATION AND USES THEREOF BACKGROUND OF THE INVENTION Thisinvention relates to apparatus for terminating enclosed wave propagationmedia, such as waveguide and coaxial cable.

While a great variety of terminations are known for waveguides, thecommonly used terminations are at least one-quarter wavelength long inorder to provide negligible reflection into the guide and to assure goodbandwidth.

Nevertheless, in some applications for terminations, compactness may bemuch more desirable than broad bandwidth. For example, in some parts ofmicrowave communication systems carrying only relatively narrow bands offrequencies, it is desired to provide isolation between components in arelatively compact space. Typically heretofore so-called resonantisolators have been used in these instances.

ln modernizing such systems, it is now frequently desirable to replaceresonant isolators with terminated circulators, since the circulatorstructures are less costly when manufactured in quantity, yield higherperformance in low power applications, and have other uses as well. Someimprovement in bandwidth over that of a resonant isolator is alsodesired. The termination of the circulator that makes it an isolatormust be compact to permit replacement of isolators in existing systemsand reduction of volume of new systems.

SUMMARY OF THE INVENTION According to my invention, a compacttermination is provided for an enclosed wave propagation medium such asa waveguide or coaxial cable by loading a section of the mediumextending to a reflective end memher with absorptive material shapedinto flrst and second portions having impedances lower than thecharacteristic impedance of the unloaded medium, the loading beingaccomplished in less than a quarter wavelength in that the secondportion is intermediate the first portion and the reflective end memberand has a value of impedance intermediate between the impedance of thefirst portion and the characteristic impedance of the unloaded medium.

Subsidiary features of the invention relate to techniques forproportioning respective impedances and electrical lengths of theportions to provide negligible reflection into the unloaded medium,relate to means for providing superior heat-sinking and further relateto application to coaxial cable terminations.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of myinvention will become apparent from the following detailed descriptiontaken together with the drawings in which:

FIG. I is a pictorial cross-sectional view of a terminated circulator orisolator using my invention;

FIGS. 2 and 3 show different views of the termination itself;

FIG. 4 schematically indicates the various impedances and lengthspertinent to the proportionings of the absorptive material of thetermination;

FIG. 5 through 9 show polar admittance diagrams, known as Smith charts,for various proportionings of the parameters indicated in FIG. 4;

FIG. 10 shows a modification of a microwave termination for waveguideshaving superior power handling capabilities; and

FIG. 11 is a pictorial cross-sectional view of a coaxial cabletermination according to my invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT In FIG. 1 the microwavewaveguideisolator II includes the circulator I3 and the termination 12.While the circulator shown is a symmetrical three port, the subsequentdiscussions apply equally to any three-part circulator. The wavepropagation path through the circulator 13 is generally from top tobottom through the input port 14 past the core element 19 and polepieces 18, then out the output port 15. The circulator 13 is completedby a third port 16, to which termination 12 is attached.

The termination 12 that makes the circulator 13 into an isolator 11 mustbe fairly compact in order to minimize the lateral protrusion of theapparatus from the wave propagation path. In the present instance, thetermination 12 is a section of loaded waveguide less than one-quarterwavelength long. It includes the waveguide sidewalls 24 and reflectiveend plate 25, as well as the absorptive element 20 which provides theloading.

Absorptive element 20 has its smallest diameter portion 22 adjacentreflective end plate 25 and its largest diameter portion 21 farthesttherefrom. It typically has a copper core 23 which may include the meansfor fastening it to reflective end plate 25. Alternatively, the core maybe deleted and the fastening may be by adhesive. It will be noted thatthe direction of stepping of the diameter of reflective element 20 isthe reverse of that of broadband microwave terminations. This fact isthe key to the compactness of element 20.

Element 20 is shown in FIG. 2 demounte'd from circulator 13 so that itmay be appreciated that it could also be used to terminate any waveguideor any waveguide stub. Such a termination would be relativelynarrowband, but would have at least the bandwidth of current or presentmicrowave communications bands. The view of termination 12 as shown inFIG. 3 simply illus trates that it has advantageously a cylindricalcrosssection in both parts, even in rectangular waveguide. Nevertheless,termination 12 could also have a rectangular or other non-cylindricalcross-section.

For a discussion of the proportions of element 20, reference is made tothe schematic diagram of FIG. 4 in which the characteristic impedance ofthe unloaded waveguide is designated Z the impedance Z is assigned tothe length of guide occupied by portion 21 and Z to the length of guideoccupied by portion 22. For a central element 20, impedance is inverselyrelated to diameter. Z, is the lowest of the three impedances; and Z isintermediate the values of Z and Z Z and Z are actually loaded waveguideimpedances associated with the respective portions of element 20. Sincethe polar admittance diagrams start at the reflective end plate 25,which has infinite admittance, the length 1 is assigned to element 22and the length 1 is assigned to element 21.

Referring now to the polar admittance diagram of FIG. 5, one sees thatconductive values of the ratio of admittance to characteristicadmittance at the point in question are plotted on the vertical diameterof the chart with a unity admittance ratio (match) at the center.Capacitive admittance ratios are to the right in the chart and inductiveadmittance ratios are to the left. Reflective end plate 25 of FIG. 1 isrepresented by the point at the bottom of the vertical diameter of thediagram, since it represents infinite admittance.

Let us assume that the ratio of Z,to Z is one-half and that the ratio ofZ 'to Z is four. These are logical assumptions since the largest portion21 will have the lowest impedance. Consider now that an electromagneticwave is propagating from reflective end plate 25 along the axis of theguide along element 22 toward element 21. The dissipative material ofelement 22 has a constant loss per unit length in terms of dB at a givenwavelength. This is represented in the Smith chart of FIG. 5 by theso-called return loss curve B which is illustr'atively characterized bya 24 l/)\ dB loss. The admittance at any given point as we move awayfrom end plate 25 lies at a point on this curve. When we reach element21 after traversing distance I, subtending an angle 720 I /A weencounter a discontinuity. On the diagram, this discontinuity isrepresented by the segment 32. The beginning and end points of segment32 are the admittances at the discontinuity normalized on impedances Z 2and Z. respectively. Since Z,/Z /2, the normalized values of conductanceand susceptance at the end point of segment 32 will be just one-half ofthe values at its beginning point.

As the electromagnetic wave moves along element 21 toward the unloadedguide, the impedance at any point is represented by curve segment 33,which lies on another constant loss per unit length curve that could beinterpolated between curves A and B.

Finally at the outer edge of element 21, an impedance discontinuity isencountered which requires renormalization from impedance value Z, tothe impedance value Z as indicated by straight line segment 34. In otherwords, the normalized conductance and susceptance at the beginning ofsegment 34 are multiplied by four which is the ratio Z /Z, to get thecoordinates of the end point of segment 34.

If we have properly proportioned the impedances and the lengths l and wewill have a match and the end point of 34 will fall at the origin of thechart. If we have not, the final segment 34 willterminate at a pointaway from the origin indicating a mismatch. In general the idealproportions depend upon frequency thus limnew termination and tends toindicate in part the range through which some of the parameters can bevaried. From FIG. to FIG. 6, the length 1, is illustratively increasedwithout changing the ratio ofZ, to Z which illustratively still remainsone-half. Thus the line segment 41 covers a greater arc length now,about 0.16 of a wavelength, and the line segment 42 representing thediscontinuity between portions 21 and 22 terminates on normalized valuesof conductance and susceptance which are about one-half the values ofthe previous example. The length 1 corresponding to the are that theline segment 43 spans is now reduced so that the admittance at its outeredge is purely conductive. The final line segment 44 represents thediscontinuity between impedance Z, and the characteristic impedance Z,of

the unloaded guide. In this case, in order for the termination to bematched, the impedance ratio Z /Z must be somewhat greater than 5:1.

It will be seen that for any set of reasonable ratios 2, to Z and Z to 2where z, Z Z lengths 1 and 1 can be chosen to provide a purely resistiveimpedance of termination 12 with a sum of I, and 1 that is less thanone-quarter of a wavelength (180around the polar impedance diagram),since the remaining part of the required lrotation through the Smithchart will always be spanned electrically by the discontinuityrepresented by the line segment 42. In other words, that discontinuityhas effective electrical length but not physical length along the guide.The reason for the shortness of the termination of the present inventioncan be explained in terms of the polar impedance diagram by noting thatprior art tapered or stepped terminations plotted on the polaradmittance diagram involve an ever-tightening spiral about the origin orsuccessive rotations and admittance transformations on the real axis (noangular change). Termination polar admittance plot always spans justhalf the impedance chart, the inductive half, and if properly designed,the element 20 has a plot that ends at the center of the chart, unityadmittance ratio, indicating a proper match.

In the above discussion the admittance of the abrupt discontinuities ofthe waveguide or load cross-section (end effects) have not been takeninto account. Since these admittances have a significant effect on thedesign and since they make determination of the-loaded guide impedancesdifficult, the design is carried out by trial and error modification ofthe lengths and the diameters of sections 21 and 22. The number oftrials re quired to obtain the final dimensions may be significantlyreduced through the use of Smith chart deviation experiments of the typeindicated in FIGS. 7, 8 and 9. These experiments show the qualitativeeffects of variations in the various parameters and serve as a guide towhat parameter should be adjusted and in what direction the adjustmentshould be made.

As a specific example, one termination has been built and tested for usein terminating a circulator as in FIG. 1. The waveguide is of a typeknown as WR l 59, atypical 6 gigahertz waveguide having internaldimensions 1.590 X 0.795 inches, the absorptive material of element 20is a resistive resin-type polymer including some iron commerciallyavailable as Emerson and Cummings MFl 17. The diameter of smallerportion 22 is 0.037 inches, the larger diameter is 0.629 inches, is0.125 inches and I is 0.175 inches. The operating frequency range is5.865 to 6.425 gigahertz. Therefore, the total termination length of0.300 inches is only oneeighth of a guide wavelength. The measuredreturn loss of the termination over this 9 percent band is greater than27 dB. When the circulator is tuned to match the termination theisolation is greater than 30 dB. A termination return loss of 25 dB overa 12-15 percent band is feasible.

In the higher power termination of FIG. 10 the termination 71 may beviewed simply as a loaded section of the waveguide 75 in which the lossymaterial is applied to the sidewalls of sections 73 and 74 of the guide.The section 73 has the lowest impedance because of the smallest lateralseparations from the guide axis and is followed by a section 74 next tothe reflective end plate which has an intermediate impedance valuebecause of intermediate separations of the guide walls from the guideaxis. As in FIG. 1 the intermediate impedance portion 74 is intermediatein position between the reflective end plate 77 and the other portion73.

The heat-sink 76 is then placed in thermal contact with walls 75 aboutthe loaded section of the guide in order to carry away the heatgenerated by dissipation of microwave energy in the lossy sections 73and 74. Note that in this case the lossy body or termination load is intwo separated sections 73 and 74.

In FIG. 11 the principles of the present invention are extended totermination 81 for coaxial cable. The coaxial cable includes the outerconductor 85 and an inner conductor 86. The absorbing element 82 is acylinder of dissipative material in contact with the outer conductor 85portions 83 and 84 of and extending over the combined lengths of theenlarged center conductor 86. The portion 83 isenlarged more thanportion 84, which is intermediate portion 83 and the end of the guide sothat the coaxial cable section including portion 84 will have a value ofimpedance intermediate the impedance of the unloaded guide and theimpedance of the coaxial cable section including portion 83. Theprinciples of operation are identical to those explained above; andtermination characteristics can be similarly plotted on a Smith chart.Such a coaxial cable termination can be used, for instance, to terminateone part of a coaxial circulator used as an isolator or to terminate anunused part of a four part directional coupler. The relatively largecontact area between the dissipative element 82 and the outer conductor85 provides for effective conduction of heat out of the dissipativematerial. For higher power applications a heat sink is easily applied,or the substantial heat-sinking capability already present can beaugmented by forced air or water flow over the exposed surface of theouter conductor.

I claim:

1. Apparatus for terminating an enclosed wave propagation medium havinga characteristic impedance, comprising a section of said medium, areflective end member for said section, and means for loading saidsection, including a bodycomposed of material absorptive for thepropagating waves and shaped into first and second portions providingsaid loaded section with respectively associated impedances lower thansaid characteristic impedance and having respective lengths in thedirection of wave propagation presenting resistive impedance at theinput to said section, the body being rendered shorter than a quarterwavelength in that the Second a r ic" interm d e sa xstiqo ti nd. saidend member and provides said loaded section with a respectivelyassociated impedance intermediate the impedance provided by said firstportion and said characteristic impedance.

2. Apparatus according to claim 1 including a circulator structurehaving a loss port, the loaded section of wave propagation medium beingcoupled to said loss port, whereby said apparatus comprises an isolator.

3. Apparatus according to claim 1 in which the impedances and lengths ofthe first and second portions of the loaded section are proportioned tomake the resistive impedance presented at the input of the loadedsection equal to the characteristic impedance of the wave propagationmedium.

4. Apparatus according to claim 1 in which the section of enclosed wavepropagation medium comprises a section of hollow metallic waveguide andthe body composed of absorptive material is positioned in contact withthe end member and spaced from the sides of the metallic waveguide, thesecond portion of said body having smaller transverse dimensions thanthe first portion of said body.

5. Apparatus according to claim 4 in which the transverse dimensions ofthe body are stepped from said first portion to said second portion.

6. Apparatus according to claim 4 in which the body has a heat-sink corecontacting the end member.

7. Apparatus for terminating an enclosed wave propagation medium havinga characteristic impedance, comprising a section of said medium, areflective end member for said section, and means for loading saidsection, including first and second hollow bodies composed of materialabsorptive for the propagating waves and shaped to provide said loadedsection with first and second respectively associated impedances lowerthan said characteristic impedance, said bodies having respectivelengths in the direction of wave propagation presenting resistiveimpedance at the input to said section, the loaded section beingrendered shorter than a quarter wavelength in that the second body isintermediate the first body and the end member and provides said loadedsection with larger lateral dimensions than does the first body and arespectively associated impedance intermediate the respectivelyassociated impedance provided said section by said first body and thecharacteristic impedance.

8. Apparatus according to claim 7 including a heat sink thermallycoupled to said bodies through both the sides of said section and thereflective end member.

9. Apparatus for terminating a coaxial line wave propagation mediumhaving a center conductor and a characteristic impedance, comprising asection of said coaxial line wave propagation medium, a reflective endmember for said section, and means for loading said section, includingfirst and second portions of the center conductor having respectivefirst and second diameters larger than the diameter associated with saidcharacteristic impedance and a body of absorptive material coaxiallyformed about said first and second portions, the second portion of thecenter conductor being between said first portion and the reflective endmembers and having smaller lateral dimensions than said first portion.

10. Apparatus according to claim 9 in which the body composed ofabsorptive material has substantial thermal coupling to the sidewallsand to the reflective end member.

11. Apparatus according to claim 9 in which the body of absorptivematerial comprises a cylindrical annulus of the absorptive material incontact with the outer conductor of the coaxial line medium andextending over the combined lengths of the first and second portions ofthe center conductor.

12. Apparatus according to claim 11 in which the outer conductorprovides significant heat-sinking capability in the vicinity of theabsorptive cylindrical annulus.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 8Ll6,720 Dated November 5, 197

Inventor(s) Theodore w Mohr It is certified that error appears in theabove-identified patent and that said Letters Patent are hereby'corrected as shown below:

Column 3, line50 z 2 A0.19T should read z w W .l9

Column line l, restore "z" (twice) to larger size.

Signed and sealed this 7th day of January 1975.

(SEAL) Attest: I

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents USCOMM-DC 60376-P69' i u.s. GOVERNMENT PRINTING OFFICE: I9l90-306-334.

FORM PO-IOSO (10-69)

1. Apparatus for terminating an enclosed wave propagation medium havinga characteristic impedance, comprising a section of said medium, areflective end member for said section, and means for loading saidsection, including a body composed of material absorptive for thepropagating waves and shaped into first and second portions providingsaid loaded section with respectively associated impedances lower thansaid characteristic impedance and having respective lengths in thedirection of wave propagation presenting resistive impedance at theinput to said section, the body being rendered shorter than a quarterwavelength in that the second portion is intermediate said first portionand said end member and provides said loaded section with a respectivelyassociated impedance intermediate the impedance provided by said firstportion and said characteristic impedance.
 2. Apparatus according toclaim 1 including a circulator structure having a loss port, the loadedsection of wave propagation medium being coupled to said loss port,whereby said apparatus comprises an isolator.
 3. Apparatus according toclaim 1 in which the impedances and lengths of the first and secondportions of the loaded section are proportioned to make the resistiveimpedance presented at the input of the loaded section equal to thecharacteristic impedance of the wave propagation medium.
 4. Apparatusaccording to claim 1 in which the section of enclosed wave propagationmedium comprises a section of hollow metallic waveguide and the bodycomposed of absorptive material is positioned in contact with the endmember and spaced from the sides of the metallic waveguide, the secondportion of said body having smaller transverse dimensions than the firstportion of said body.
 5. Apparatus according to claim 4 in which thetransverse dimensions of the body are stepped from said first portion tosaid second portion.
 6. Apparatus according to claim 4 in which the bodyhas a heat-sink core contacting the end member.
 7. Apparatus forterminating an enclosed wave propagation medium having a characteristicimpedance, comprising a section of said medium, a reflective end memberfor said section, and means for loading said section, including firstand second hollow bodies composed of material absorptive for thepropagating waves and shaped to provide said loaded section with firstand second respectively associated impedances lower than saidcharacteristic impedance, said bodies having respective lengths in thedirection of wave propagation presenting resistive impedance at theinput to said section, the loaded section being rendered shorter than aquarter wavelength in that the second body is intermediate the firstbody and the end member and provides said loaded section with largerlateral dimensions than does the first body and a respectivelyassociated impedance intermediate the respectively associated impedanceprovided said section by said first body and the characteristicimpedance.
 8. Apparatus according to claim 7 including a heat sinkthermally coupled to said bodies through both the sides of said sectionand the reflective end member.
 9. Apparatus for terminating a coaxialline wave propagation medium having a center conductor and acharacteristic impedance, comprising a section of said coaxial line wavepropagation medium, a reflective end member for said section, and meansfor loading said section, including first and second portions of thecenter conductor having respective first and second diameters largerthan the diameter associated with said characteristic impedance and abody of absorptive material coaxially formed about said first and secondportions, the second portion of the center conductor being between saidfirst portion and the reflective end members and having smaller lateraldimensions than said first portion.
 10. Apparatus according to claim 9in which the body composed of absorptive material has substantialthermal coupling to the sidewalls and to the reflective end member. 11.Apparatus according to claim 9 in which the body of absorptive materialcomprises a cylindrical annulus of the absorptive material in contactwith the outer conductor of the coaxial line medium and extending overthe combined lengths of the first and second portions of the centerconductor.
 12. Apparatus according to claim 11 in which the outerconductor provides significant heat-sinking capability in the vicinityof the absorptive cylindrical annulus.